Power line faults can be detected and remedied through the use of solid state fault current limiters (SSFCL). These SSFCL devices utilize solid state switching devices, such as IGBT, SCR, IGCT or MOSFET transistors, to block or significantly increase the impedance of the current path between the power source and the load. However, these power lines typically carry voltages ranging from 10 kV to over 230 kV. Since the typical switching device is only rated to roughly 6 kV, often it is necessary to place a plurality of these SSFCL devices in series. The total voltage of the power line, also referred to as the power line voltage, is divided across the total number of SSFCL devices in series, thereby allowing each to operate within its rated range. When a fault is detected, such as a surge in current through the power line, each of the SSFCL devices disables its respective solid state switching device, thereby increasing the resistance seen by the power source and lowering the current.
FIG. 1 shows a SSFCL device 100 commonly used. The SSFCL device 100 comprises a solid state switching device 110, which may be any of the transistors described above. These solid state switching devices 110 typically have at least three terminals, a source or input side 111, a drain or output side 112, and a gate 113. The assertion of the gate 113 allows the passage of current from the source 111 to the drain 112, while the deassertion of the gate 113 disables the passage of current through the solid state switching device 110.
This solid state switching device 110 may be in parallel with one or more of the following components: a snubber 120, a reactor 130 and a transient suppressor 140. The snubber 120 is typically a resistor in series with a capacitor used to dissipate the energy of the transient and to reduce the overvoltage by filtering the transient frequency (i.e. slowing the “ringing” frequency). The transient suppressor 140 is used to clamp the overvoltage transient below the level of the ratings of the snubber 120 and solid state switching device 110. The snubber 120, the reactor 130 and the transient suppressor 140 may be referred to as parallel components 145, since these components 145, in some embodiments, provide a parallel path for current for travel when the solid state switching devices 110 are in the disabled or off state. These parallel components 145 are used to provide an alternative high impedance current path from the power source to the load when the solid state switching device 110 is in the off state and protect the solid state switching device 110 from transient overvoltage during turn on and turn off.
The gate 113 of solid state switching device 110 is in communication with a gate driving circuit 150. This gate driving circuit 150 monitors the current being supplied by the power line 101 using a current sensor 160. The gate driving circuit 150 is used to enable or block the passage of current through the solid state switching device 110, based on information from the current sensor 160.
The gate driving circuit 150 may be referenced to the voltage seen by the solid state device 110. In other words, its output voltage is related to the voltages presented on the source 111 or drain 112 of the solid state switching device 110. Traditionally, this is achieved by using an isolated power supply 170. This DC power supply 170 may be a relatively low voltage, low current power supply. For example, the gate driving circuit 150 typically utilizes low voltage, such as up to 48V, and dissipates only a few watts.
However, the isolated power supply 170 of each SSFCL 100 must be electrically isolated from every other isolated power supply 170. In some embodiments, the magnitude of the isolation voltage must be at least the total line voltage divided by the number of SSFCL devices 110. In other embodiments, the magnitude of the isolation voltage must be at least the total line voltage.
This isolation is typically performed using an isolated DC power source 170. These isolated DC power sources 170 may be optically isolated, or isolated using another means. In these embodiments, despite the low voltage and current requirements, the isolated DC power supply 170 may be unreliable and very expensive, potentially costing thousands of dollars each. These isolated power supplies 170 have to deliver stable power over the isolation rated at high voltage. The higher the isolation voltage, the more difficult this task becomes, as the size of the supplies will grow, the cost will grow, the reliability will decrease due to higher probability of high voltage breakdown causing insulation puncture. The lower reliability may also be due to the fact that the output voltage regulation will be difficult to control from the high voltage side (it would have to be controlled on the ground side) and it may be difficult to maintain the voltage needed by the gate of the switch to turn it on and off. Thus, the need to use this specialized isolated power supply 170 greatly increases the total cost of a state solid fault current limiter system.
Therefore, it would be beneficial if there were a system and method for providing isolated power to the gate driving circuits that was less expensive and more reliable than current solutions.